Electron spin resonance imaging (ESR imaging) is a technique for obtaining spatially resolved ESR-based data from samples. ESR-based data is data obtained by applying on the sample a static magnetic field, exciting electron spins in the sample by electromagnetic waves, and detecting irradiation created by the electron spins as a result of their excitation.
The spatially resolved ESR-based data can, for example, provide information about chemical structure, eluci date biological functions, enable non-invasive medical diagnosis, and be used to solve material science problems.
In classical ESR detection method, termed “induction detection”, the imaged specimen is put inside a microwave resonator, and is subjected to a static magnetic field and to microwave radiation. Within the framework of “induction detection”, there are two possible schemes to obtain the ESR-based data, pulsed and continuous wave ESR. In pulsed ESR, a specific set of microwave pulses are induced upon the sample and after a short while the spins in the sample create microwave radiation of their own, termed “echo”, which is concentrated by the resonator and analyzed. In continuous wave (CW) ESR, the sample is irradiated with a continuous microwave irradiation, and the ESR signal is detected by monitoring the microwave signal reflected from the resonator.
To obtain spatial information on the location of the spins that create the ESR signal, one can make use of the fact that the frequency of the ESR signal changes with the intensity of the magnetic field. Specifically, a combination of static and time varying magnetic field gradients are applied across the specimen, such that each point (voxel) in the specimen is exposed to a magnetic field with different intensity at different times. This way, spins from each voxel irradiate in a different frequency or with different phase, and the frequency or phase is used to determine the location of the spins that created the signal.
The magnetic field gradients are usually applied with gradient coils. The stronger is the gradient provided by the gradient coils, the better is the spatial resolution of the obtained ESR image.
An imaging device, designed to image a specimen when the specimen is inside the imaging device, is said to have in-situ geometry. An imaging device, designed to image a specimen when the specimen is outside the imaging device is said to have an ex-situ geometry. In the context of induction ESR, a specimen is said to be inside the imaging device if it is inside the smallest is convex volume that includes the resonator, the gradient coils, and the static magnetic field source. Applicant is not aware of any ESR imaging device, which operates in the induction detection method and has ex-situ geometry.
One drawback of the in-situ geometry is the need to insert the specimen into the imaging device. This limits the size of the specimen, or requires a large device.
A drawback of induction detection is its notoriously low Signal-to-Noise-Ratio (SNR), which requires, under ambient conditions, to sense at least 107 spins per voxel. These SNR limitations are also reflected by the limited image resolutions currently achievable with induction detection-based electron spin resonance imaging.
The best available resolution for induction detection was achieved in a system described in US patent application publication No. 2006-0022675 (hereinafter US '675), wherein the present inventor is a co-inventor. This publication describes room temperature in-situ ESR imaging probe, for biological applications, having a resolution of the order of (1 μm)3.
In recent years, new ESR detection methods were introduced, which, under extreme physical conditions, or for unique samples, can greatly improve the sensitivity and the image resolution of magnetic resonance. These methods include, for example, Scanning Tunneling Microscopy-ESR (STM-ESR), Magnetic Resonance Force Microscopy (MRFM), Hall detection, Optically Detected Magnetic Resonance, quantum dot spin detection, and electrically detected magnetic resonance.
This wide variety of techniques, however, has very limited spectroscopic and 3D imaging capability, and often requires very complicated sample preparation procedures, which makes its practical use very difficult.
WO2005/073695 describes an on-chip magnetic resonance spectroscopy in ex-situ geometry, but does not provide imaging possibilities, and does not operate in the induction detection method.
WO02/21147 describes an ex-situ microwave microscope capable of detecting ESR signals using induction detection, but at very low resolution and without ex-situ static magnetic field source and gradients.